Messier 56 (M56)

The Hubble Space Telescope produced this image of the globular cluster Messier 56 (also known as M56 or NGC 6779). The cluster is composed of a large number of stars, tightly bound to each other by gravity. Astronomers typically infer important properties of globular clusters by looking at the light of their constituent stars. But they have to be very careful when they observe objects like Messier 56, which is located close to the galactic plane. This region is crowded by "field stars," in other words, stars in the Milky Way that happen to lie in the same direction but do not belong to the cluster. These objects can contaminate the light, and hence undermine the conclusions reached by astronomers. A tool often used by scientists for studying stellar clusters is the color-magnitude (or Hertzsprung-Russell) diagram. This chart compares the brightness and color of stars, which, in turn, tells scientists what the surface temperature of a star is. By comparing high-quality observations taken with the Hubble Space Telescope with results from the standard theory of stellar evolution, astronomers can characterize the properties of a cluster. In the case of Messier 56, this includes its age, which at 13 billion years is approximately three times the age of the Sun. Furthermore, they have been able to study the chemical composition of Messier 56. The cluster has relatively few elements heavier than hydrogen and helium, typically a sign of stars that were born early in the universe's history, before many of the elements in existence today were formed in significant quantities. Astronomers have found that the majority of clusters with this type of chemical makeup lie along a plane in the Milky Way's halo. This suggests that such clusters were captured from a satellite galaxy, rather than being the oldest members of the Milky Way's globular cluster system as had been previously thought.